Recurrent or chronic abdominal pain is one of the common gastrointestinal problems in children. While most children do not have organic origins (so called functional abdominal pain disorders; FAPDs), the symptoms can nevertheless be severe enough to impair the patient's quality of life, growth, and development. To help rule out organic disorders and diagnose this condition, some individuals underwent multiple invasive and costly studies. Generally, the diagnosis of FAPDs is based on clinical symptoms and criteria, it can be divided into irritable bowel syndrome (IBS), abdominal migraine, functional abdominal pain (FAP) and functional dyspepsia (FD). Approximately 14% of children globally, between the ages of 4 and 18, experience functional abdominal pain issues8. In Thailand, the prevalence of FAPDs among adolescents (mean age of 16 years) was 5.3%, functional dyspepsia and irritable bowel syndrome were found to be the most prevalent subtypes. The pathogenesis of FAPDs is believed to result from disruptions in the microbiota-gut-brain axis, which may happen early in life or throughout. Hence, several studies, specifically in western countries, reported the role of probiotics, specifically Lactobacillus rhamnosus GG (LGG), in modulating abdominal symptoms in children with FAPDs. It is widely known that the diversity of gut microbiota depends on multiple factors including ethnicity, mode of delivery, dietary and environmental factors. However, the studies on the use of probiotics in pediatric patients with FAPDs have been mainly conducted in western countries. Since there are limited studies on the effectiveness of probiotics in Asian children with FAPDs, the investigators aim to evaluate the effects of probiotics, LGG, in the treatment of children who suffered from FAPDs. The secondary objectives are to measure daily pain score in children with and without FAPDs, to evaluate and compare the diversity of fecal microbiota in children with FAPDs and those without FAPDs, and to compare the diversity of fecal microbiota between children with FAPDs who took probiotics and those who did not.